ehsvp, not sure if this is applicable as a reply but I've included sealed voids in welded aluminum construction since the 1970's in boats of various sizes I've designed and built. A sealed void in metal will provide some tremendous forces in the boat's structure if not vented to allow daily temperature swings to adjust the air volume inside a void.

Moving to the case of a sealed deck welded to the sides of the hull so the volume below the deck is filled with air but water can't get in- those bilge areas should be at the least vented to allow air volume adjustment or... the bilge will be under pressure if the deck is warmed in the sun or vacuum is the boat is on the water when the sun goes down or even behind a cloud!

I use a 1/2 NPT coupling welded to the deck, usually in the after corners where there is a locker or low traffic, but I have done the deck vent inside a centered, free standing console too. I used SS industrial tubing and matching fittings to create as tall a 'goose neck' as the gunwale or console allows. I don't vent the tubing overboard just into the space below the sheer clamp/guard deck/gunwale plate or below the console top.

I don't consider any tank installation that is properly designed, built and installed with fuel proof hose as even a remote possibility of requiring a separate 'fumes' or vapors vent. (Although Capt'n Dave had just such a mishap recently posted here for review: I considered that the result of poor design and lack of provision for owner maintenance- not normal use but a system failure due to inadequate design provisions BEFORE the build)

If, for any reason, I was involved with a tank installation that required a separate fume or vapor vent from the tank being inclosed in a sealed void: I would personally think it was necessary to create a dedicated tank only volume- a separate and dedicated volume inside the bilge or other locations- dedicated to secondary containment of the tank's potential fugitive emissions.

If I put a tank under a deck, I don't think it will need space venting for hydrocarbon vapors? I do vent decks with open tubing but that is to accommodate temperature driven volume changes - not gas fumes.

Not sure I've addressed your question? If not? please feel free to restate and show a little more of your thinking so I could try to address the point a little better?

Kevin, thanks for the reply, sorry it took so long to get back to you. My question was answered perfectly and I will do the venting of the bilge area of my boat as you have suggested. I will post some pictures of the boat when the painting is done in spring. Thanks again,
Bob